In recent years, the use of chemical leaching to recover minerals from low grade mineral bearing ores has grown. For example, caustic cyanide leaching is used to recover gold from low grade ores having about 0.02 ounces of gold per ton and dilute sulfuric acid is used to recover copper from copper bearing ores. Such leaching operations are typically carried out in large heaps. The mineral bearing ore from an open pit mine, or other source, is crushed to produce an aggregate that is coarse enough to be permeable in heaps but fine enough to expose the mineral values to the leaching solution. After crushing, the ore is formed into heaps on impervious leach pads. The leaching solution is evenly distributed over the top of the heaps by sprinklers, wobblers, or other similar equipment. As the barren leaching solution percolates through the heap, it dissolves the minerals contained in the ore. The liquor collected by the impervious leach pad at the bottom of the heap is recovered and this "pregnant solution" is subjected to a mineral recovery operation. The leachate from the recovery operation is held in a barren pond for reuse.
Economical operation of such heap leaching operations requires that the heaps of crushed ore have good permeability after being crushed and stacked so as to provide good contact between the ore and the leachate. Ores containing excessive quantities of clay and/or fines (i.e., 30% by weight of .sup.- 100 mesh fines) have been found undesirable due to their tendency to slow the percolation flow of leach solution. Slowing of the percolation flow of the leach solution can occur when clay and/or fines concentrate in the center of the heap while large rock fragments tend to settle on the lower slopes and base of the heap. This segregation is aggravated when the heap is leveled off for installation of the sprinkler system that delivers the leach solution. This segregation results in localized areas or zones within the heap with marked difference in permeability. The result is channeling where leach solution follows the course of least resistance, percolating downward through the course ore regions and bypassing or barely wetting areas that contain large amounts of clay and/or fines. Such channeling produces dormant or unleached areas within the heap. The formation of a "slime mud" by such fines can be so severe as to seal the heap causing the leach solution to run off the sides rather than to percolate. This can require mechanical reforming of the heap. The cost of reforming the heaps which can cover 160 acres and be 200 feet high negates the economies of scale that make such mining commercially viable.
In the mid 1970s, the United States Bureau of Mines determined that ore bodies containing high percentages of clay and/or fines could be heap leached if the fines in the ore could be agglomerated. The Bureau of Mines developed an agglomeration process in which crushed ore is mixed with Portland cement at the rate of from 10 to 20 pounds per ton, wetted with 16 to 18% moisture (as water or leach solution), agglomerated by a disk pelletizer and cured for a minimum of eight hours before being subjected to stacking in heaps for the leaching operation.
In commercial practice, the method developed by the United States Bureau of Mines has not met with widespread acceptance because of the cost and time required. However, the use of cement, as well as other materials, as agglomerating agents is known. Agglomerating practices tend to be site specific and non-uniform. Typically, the action of the conveyors which move the ore from the crusher to the ore heaps or the tumbling of ore down the conical piles is relied on to provide agglomeration for a moistened cement-ore mixture. Lime has been found to be less effective than cement in controlling clay fines, it is believed this is because the lime must first attack the clay lattice structure in order to provide binding.
Cement has been found to be most effective in high siliceous ores (crushed rock) and noticeably less effective in ores having a high clay content. The large volumes of cement required also present problems. The transportation to and storage of large volumes of dusty cement at the often remote mine locations is difficult. With the growth of such mining methods, the need for cost effective, efficient agglomerating materials has grown.
U.S. Pat. Nos. 5,077,021 and 5,077,022 disclose agglomerating agents and methods for use in heap leaching which comprise anionic polymers of acrylamide and acrylic acid.
U.S. Pat. No. 4,875,935 discloses a method for extracting copper from copper minerals which employs an agglomerating agent comprising anionic acrylamide polymers containing at least five mole percent of carboxylate or sulfonate groups. U.S. Pat. Nos. 4,898,611 and 5,100,631 disclose improvements in the agglomeration of a gold or silver ore with cement comprising including specific water soluble vinyl polymers in the cement agglomeration treatment.
U.S. Pat. Nos. 5,112,582 and 5,186,915 disclose agglomerating agents and methods for using heap leaching of mineral bearing ores which comprises an anionic polymer of acrylamide and acrylic acid and sufficient time to provide a pH of from about 9.5 to 11.